The present invention relates to an exhaust gas aftertreatment device for internal combustion engines having a catalyzer for selective catalytic reduction of oxides of nitrogen from exhaust gases, especially from exhaust gases of motor vehicle diesel engines. More particularly, the present invention relates to a device comprising a metering appliance for an overstoichiometric supply of one of NH.sub.3 and materials releasing NH.sub.3, at least two sensors, one of which is an NH.sub.3 sensor configured to interrupt the supply when a quantity of NH.sub.3 quantity exceeds a specified upper threshold value, and to resume the supply when, in the catalyzer, a stored NH.sub.3 quantity reaches a specified lower threshold value, and also to a device comprising a metering appliance for supply of one of NH.sub.3 and of materials releasing NH.sub.3, and a single sensor for determining the NH.sub.3 concentration in the exhaust gas.
As is well known, the oxides of nitrogen contained in the exhaust gases are reduced to nitrogen and water on a catalyzer when a reducing agent, namely ammonia (NH.sub.3) or compounds forming ammonia, is added.
Measures are disclosed in DE 3,825,206 to provide a pulsed overstoichiometric metered addition of the reducing agent NH.sub.3 and, specifically, by measuring the NO.sub.x concentration before and after the catalyzer, because the charge condition of the catalyzer is not defined in this mode of operation.
Furthermore, measures for the selective catalytic reduction of oxides of nitrogen from exhaust gases are described in German Patent Application P 41 17 143.8-43, by way of which the high NH.sub.3 concentration occurring in the metering phase is recorded by a sensor placed in the catalyzer. The sensor interrupts the NH.sub.3 supply after detection of the specified NH.sub.3 concentration. As soon as the NH.sub.3 stored in the catalyzer is substantially used up by the reaction, the renewed employment of the NH.sub.3 supply is determined by approximate calculation, from the engine characteristic diagram and the operating period, of the NO.sub.x produced by the engine over the period since the beginning of metering or the end of metering, taking account of the average degree of separation.
An object of the present invention is to provide simple measures In an exhaust gas aftertreatment device provided for non-stationary internal combustion engines to further improve the reduction of the oxides of nitrogen contained in the exhaust gas.
The foregoing object has been achieved in accordance with the present invention by providing a further sensor as an NH.sub.3 sensor recognizing the lower threshold value of the stored NH.sub.3 quantity, or by supplying NH.sub.3 in the gas phase without metering pauses so that the NH.sub.3 concentration recorded by the sensor is compared, as the actual value, with a required value corresponding to a specified NH.sub.3 concentration to form a correction signal usable for triggering the metering appliance continuously connected into the gas phase.
Due to the advantageous measures according to the present invention, it is not necessary to calculate the charge level in the catalyzer during the metering pause or NH.sub.3 interruption phase on the basis of the characteristic diagram. The matching of the NH.sub.3 supply to the various engine types with very different exhaust gas emissions becomes superfluous and it is not necessary to take account of the unavoidable component scatter within a type in the selection of the level limits. A second ammonia (NH.sub.3) sensor, which detects the ammonia adsorbed in the catalyzer, undertakes the recognition of the lower charge level whereas the first ammonia sensor detects gaseous ammonia.
This first NH.sub.3 sensor can be placed either downstream of the catalyzer or in the catalyzer itself. Although the arrangement of the sensor within the catalyzer does not permit an optimum utilization of the catalyzer volume, it does ensure that the NO.sub.x emissions do not exceed the permissible limiting values. The arrangement of the sensor after the catalyzer permits, however, full utilization of the catalyzer volume for the maximum adsorption capacity, but a brief minimum unallowable NH.sub.3 break-out cannot always be excluded.
Although a special embodiment with a second sensor is generally described in the aforementioned German Patent application P 41 17 143.8-43, this special configuration is subjected to exhaust gas upstream of the catalyzer in contrast to the present invention.
Continuous control of a slight, but constant NH.sub.3 slip within permissible limiting values is possible by measures in which the NH.sub.3 is supplied in the gas phase without metering pauses so that the NH.sub.3 concentration recorded by the sensor is compared, as the actual value, with a required value corresponding to a specified NH.sub.3 concentration to form a correction signal usable for triggering the metering appliance continuously connected into the gas phase. The advantage of this arrangement lies in avoiding the need for any type of characteristic diagram and in the compensation for any changes to the engine and the catalyzer within the control range.